As is known, in optical-fibre communication systems, in order to avoid conversion of optical signals into electronic signals and the subsequent conversion of electronic signals into optical signals, it is desirable to have miniaturized devices for addressing and switching optical signals. Advantageously, the addressing and switching devices must be activatable by electrical signals generated by electronics associated with the devices, which are preferably integrated.
To this aim, it has been proposed to manufacture small mobile reflecting surfaces (mirrors) employing micro-electromechanical-machining technologies used for microelectromechanical devices (MEMs). In particular, the mirrors may have just one degree of freedom (they are able to rotate about just one axis) for making two-dimensional switches, or two degrees of freedom (they are able to rotate about two axes) for making three-dimensional switches. Different processes have been proposed for making optical-switching devices of the type referred to above; however, these processes are rather complex and present some limitations.
For example, EP-A-I 180 848 describes a process for manufacturing a switch, wherein the rotation of a mirror element is obtained by converting a translational motion generated by a linear actuator through a conversion assembly or joint arranged between the mirror element and the actuator.
A further process, implemented by the present applicant, enables the manufacture of a mirror element formed by a reflecting platform and a plurality of fingers (mobile electrodes), which are biased at a potential with respect to fixed electrodes so as to cause an attraction between some of the fixed electrodes and the mobile electrodes and hence a rotation of the mirror element. According to this process, the fixed electrodes are formed in a first wafer, the mirror element with the mobile electrodes are formed in a second wafer, bonded to the first wafer and appropriately thinned, and the portion of the first wafer underneath the platform is removed from the back after bonding the second wafer to a third temporary handling wafer.
The above process is disadvantageous in that, in addition to being on the whole rather complex, it involves etching of the silicon of the first wafer throughout its thickness with the purpose of freeing the mirror element. In addition, it entails the formation of openings in the second wafer for accessing the first wafer and appropriately biasing the fixed electrodes. This is disadvantageous in that the contacts to the fixed electrodes are at a different level from the contacts to the mobile electrodes and to other possible structures, thus creating problems in the testing step. In addition, the first wafer is all at a same potential (the potential of the stator), and this may create problems after assembly on a board if there are leakages.